It is known to control the injection of fuel in each cylinder of a Diesel engine using a closed-loop control of a parameter representative of the fuel combustion in the engine cylinders, in order to stabilize the combustion and reduce polluting emission. One of the mostly used parameter in controlling the combustion of a Diesel engine is an index which represents the crank angle, at which a given mass fraction of the fuel injected in the cylinder during an engine cycle has been burnt. As a matter of fact, said index typically indicates the crank angle at which 50% of the injected fuel mass has been burnt into the cylinder, so that it is generally referred as MFB50 (Mass Fraction Burnt 50%).
The determination of such index requires the ECU to sample the pressure within the cylinder during an engine cycle, in order to acquire an in-cylinder pressure curve. The pressure is sampled by means of a pressure sensor set inside the cylinder, typically integrated in the glow plug associated to the cylinder itself.
The ECU uses the in-cylinder pressure curve for calculating a curve representing the heat release rate during said engine cycle, according to the equation:
                                          ⅆ            Q                                ⅆ            α                          =                                            k                              k                -                1                                      ·            P            ·                                          ⅆ                V                                            ⅆ                α                                              +                                    1                              k                -                1                                      ·            V            ·                                          ⅆ                P                                            ⅆ                α                                                                        (        1        )            Where Q represents the heat, P represents the in-cylinder pressure, V represents the volume of the combustion chamber defined by the piston within the cylinder, k is the specific heat ratio (the ratio between the specific heat constants for constant pressure and constant volume processes) and a represents the crank angle.
The heat release rate curve is then integrated by the ECU according to the equation:
                    Q        =                              1                          k              -              1                                ·                      ∫                                          (                                                      k                    ·                    P                    ·                                                                  ⅆ                        V                                                                    ⅆ                        α                                                                              +                                      V                    ·                                                                  ⅆ                        P                                                                    ⅆ                        α                                                                                            )                            ·                              ⅆ                α                                                                        (        2        )            in order to achieve a curve representing the cumulative heat release during the engine cycle.
At this point, the ECU determines the minimum and the maximum value of the cumulative heat release curve, and uses the given fuel mass fraction (50% in the case of MFB50 determination), for calculating a target value of the cumulative heat release between said minimum and maximum values, according to the equation:Tv=mv+f(Mv−mv)  (3)Where Tv is the target vale, my and My are respectively the minimum and maximum value of the cumulative heat release curve, and f is a fraction corresponding to the given fuel mass fraction.
Finally, the ECU finds the goal point of the cumulative heat release curve which corresponds to the target value Tv, and assumes as index the crank angle corresponding to the goal point.
A drawback of this method is that the sampled in-cylinder pressure curve may be affected by some noises due to pressure sensor wiring, or to electrical interferences between the pressure sensor and other components of the engine system, such as for example the glow plug and the actuator of the injector. These noises manifest themselves in form of variations of the pressure curve, which locally deviates from the expected trace and rapidly returns to it.
It follows that the pressure rate dP/dα in the neighborhood of said noises is quite high and therefore, according to equation (2), it produces an unexpected fluctuation in the heat release rate curve having high amplitude. Such fluctuation is further magnified if the pressure noise is located in a portion of the pressure curve corresponding to a phase of the engine cycle in which also the combustion chamber volume rate dV/dα is high. According to equation (3), each unexpected fluctuation of the heat release rate curve produces in turn a fake spike in the cumulative heat release curve, which can stick out from the expected trace either upward or downward. These fake spikes can imply several problems in the determination of the MFB50, as well as of any other index representing a crank angle at which a given quantity of injected fuel mass has been burnt into the cylinder.
A first problem consists in that the vertex of a fake spike could actually be the minimum or the maximum value of the cumulative heat release curve. In this case, the presence of the fake spike introduces an error in calculating the target value Tv, which results in a deviation of the determined index with respect to the real one.
Even if the target value Tv was correct, a second problem consists in that a fake spike could have one or more points corresponding to the target value Tv. In this case, it is generally not possible for the conventional system to effectively distinguish the goal point of the cumulative heat release curve from the points belonging to the fake spike, so that said conventional system could find a wrong goal point, which inevitably returns an index different from the real one.
In view of the foregoing, at least one object of the present invention is to solve, or at least to positively reduce the above mentioned drawbacks, in order to achieve an index which is more reliable than that provided by the conventional system. At least another object of the present invention is to meet the above mentioned object with a simple, rational and inexpensive solution. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.